Three-dimensional topological insulators constitute examples of symmetry protected topological states in the absence of applied magnetic fields and cryogenic temperatures. A token characteristic of these non-magnetic bulk insulators is the protected electronic states located on the materials' surfaces. The topological electronic bandstructure of a bulk topological insulator ensures the presence of gapless surface electronic states with Dirac-like dispersions similar to graphine. Jozwiak, C. et al. Photoelectron spin-flipping and texture manipulation in a topological insulator. Nature Physics 9, 293-298 (2013): which is hereby incorporated by reference in its entirety including the supplemental information. However, unlike graphene, electrons in the topological surface states are spin polarized with their spins regulated by their direction of travel resulting in a helical spin texture in momentum space. Hsieh, D, et al. A tunable topological insulator in the spin helical Dirac transport regime. Nature 460, 1101-1105 (2009); which is hereby incorporated by reference in its entirety. Each momentum state in a topological insulator can only host one surface electron, and since its spin is regulated by its momentum, it results in the helical spin texture. Xue, Q-K. Full spin ahead for photoelectrons, Nature Physics 9, 265-266 (2013); which is hereby incorporated by reference in its entirety.
A current method of generating an electron source is through the use of traditional photocathodes, where the photocathode is illuminated by light and electrons are ejected through the photoelectric effect. This is a popular method as short electron bunches are easy to create from photocathodes. However, for a spin-polarized electron source, the most popular technique is using the optical orientation effect in GaAs photocathodes. This includes a laser beam illuminating a GaAs wafer to eject a spin polarized electron beam. The polarization of the light controls the spin polarization of the electron beam: right circularly polarized light creates a spin-up polarized electron beam, and a left circularly polarized beam creates a spin-down beam.
This current method of GaAs photocathodes is imperfect. The laser beam must be a particular photon energy that closely matches the energy gap in GaAs, or else the spin polarization will be low. Also, the photon energy is too low to overcome the work function of GaAs and will not eject electrons from the surface of GaAs. Therefore, the surface must have a careful atomic layer of Cs and O2 applied to the surface creating a dipole layer, reducing the work function allowing for sufficient photoemission at such low photon energy levels. If this layer is produced incorrectly, very few electrons will be ejected, creating a low intensity electron source.
The production of this layer is difficult and tedious. First, the GaAs is often chemically etched and then quickly placed into an ultra-high vacuum. The GaAs surface must be cleaned in the vacuum by heating to temperatures around 550° C.; too low of a temperature and the surface will not be clean enough while too high of a temperature and the As evaporates ruining the wafer. After the GaAs surface is cleaned in the vacuum, the Cs must be deposited in-situ, followed by the correct O2 exposure. This tediously prepared surface must be kept in extreme vacuum conditions and deteriorates within several days at 1×10−11 torr.
Another issue with the layering for the GaAs photocathode method is that the spin-polarization is low. For a plain GaAs wafer with the perfect laser photon energy, the theoretical maximum: polarization is around 50%, and actual yield is closer to 25-30%. The theoretical maximum can be increased to 100% if highly specialized wafers are used in which the GaAs is artificially strained through the growth of superlattice wafers which forces the GaAs to take on a different crystalline shape. These highly specialized wafers have been shown to yield 90% polarization; however, these are much more expensive, difficult to work with, and give inconsistent results.
A third issue with the layering of the GaAs photocathode method is that the resulting spin-polarization is locked along the axis of the laser beam. This means that the spin polarization can only be made “up” or “down” perpendicular to the GaAs or longitudinally to the beam's direction of propagation. This requires electrostatic optics that can steer the electron beam's direction without affecting the spin orientation. Magnetic elements can also be introduced to further manipulate the spin orientation, but these electrostatic optics and magnetic elements all add up to further complications, added errors, and higher costs.